1. Field of the Invention
This invention relates to semiconductor materials having enhanced thermoelectric properties for use in fabricating thermoelectric devices.
2. Description of the Related Art
The basic theory and operation of thermoelectric devices has been developed for many years. Presently available thermoelectric devices used for cooling typically include an array of thermocouples which operate in accordance with the Peltier effect. Thermoelectric devices may also be used for heating, power generation and temperature sensing.
Thermoelectric devices may be described as essentially small heat pumps which follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other apparatus used to transfer heat energy. A principal difference is that thermoelectric devices function with solid state electrical components (thermoelectric elements or thermocouples) as compared to more traditional mechanical/fluid heating and cooling components. The efficiency of a thermoelectric device is generally limited to its associated Carnot cycle efficiency reduced by a factor which is dependent upon the thermoelectric figure of merit (ZT) of materials used in fabrication of the associated thermoelectric elements. Materials used to fabricate other components such as electrical connections, hot plates and cold plates may also affect the overall efficiency of the resulting thermoelectric device.
The thermoelectric figure of merit (ZT) is a dimensionless measure of the effectiveness of a thermoelectric device and is related to material properties by the following equation:
ZT=Sxcex8"sgr"T/xcexaxe2x80x83xe2x80x83(1)
where S, "sgr", xcexa, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. The Seebeck coefficient (S) is a measure of how readily the respective carriers (electrons or holes) can transfer energy as they move through a thermoelectric element which is subjected to a temperature and electric potential gradient. The type of carrier (electron or hole) is a function of the materials selected to form each thermoelectric element.
The electrical properties (sometimes referred to as electrical characteristics, electronic properties, or electronic characteristics) associated with materials used to form thermoelectric elements may be represented by S2"sgr". Many of the materials which are used to form thermoelectric elements may be generally described as semiconductor compounds or semiconductor materials. Examples of such materials will be discussed later in more detail.
The thermoelectric figure of merit is also related to the strength of interactions between the carriers and vibrations of the crystal lattice structure (phonons) and available carrier energy states. Both the crystal lattice structure and the carrier energy states are a function of the materials selected to form each thermoelectric device. As a result, thermal conductivity (xcexa) is a function of both an electronic component (xcexae) primarily associated with the respective carriers and a lattice component (xcexag) primarily associated with the respective crystal lattice structure and propagation of phonons through the respective crystal lattice structure. In the most general sense, thermal conductivity may be stated by the equation:
xcexa=xcexaxcexa9+xcexaxcex5xe2x80x83xe2x80x83(2)
The thermoelectric figure of merit (ZT) may also be stated by the equation:                     ZT        =                                            S              θ                        ⁢            T                    ρκ                                    (        3        )                                ρ        =                  electrical          ⁢                      xe2x80x83                    ⁢          resistivity                                    xe2x80x83                                σ        =                  electrical          ⁢                      xe2x80x83                    ⁢          conductivity                                    xe2x80x83                                          electrical          ⁢                      xe2x80x83                    ⁢          conductivity                =                                            1                              electrical                ⁢                                  xe2x80x83                                ⁢                resistivity                                      ⁢                          xe2x80x83                        ⁢            or            ⁢                          xe2x80x83                        ⁢            σ                    =                      1            ρ                                              xe2x80x83            
Thermoelectric materials such as alloys of Bi2Te3, PbTe and BiSb were developed thirty to forty years ago. More recently, semiconductor alloys such as SiGe have been used in the fabrication of thermoelectric devices. Commercially available thermoelectric materials are generally limited to use in a temperature range between 200K and 1300K with a maximum ZT value of approximately one. The coefficient of performance of such thermoelectric devices remains relatively low at approximately one, compared to approximately three for a mechanical device. For the temperature range of xe2x88x92100xc2x0 C. to 1000xc2x0 C., maximum ZT for many state of the art thermoelectric materials also remains limited to values of approximately 1, except for Texe2x80x94Agxe2x80x94Gexe2x80x94Sb alloys (TAGS) which may achieve a ZT of 1.2 to 1.4 in a relatively narrow temperature range. Materials such as Si80Ge20 alloys used in thermoelectric generators to power spacecrafts for deep space missions have an average thermoelectric figure of merit of approximately equal to 0.5 from 300xc2x0 C. to 1,000xc2x0 C.
Many crystalline materials with low thermal conductivity do not have good electrical conductivity and many crystalline materials with good electrical conductivity often have relatively high values of thermal conductivity. For example, many binary semiconductor compounds which have skutterudite type crystal lattice structures have relatively good electrical properties. However, the value of thermal conductivity associated with the crystal lattice structures of such semiconductor compounds is generally relatively large which often results in a thermoelectric figure of merit which is less than desired.
The terms xe2x80x9cZintl phasexe2x80x9d and xe2x80x9cZintl compoundxe2x80x9d are often used to describe intermetallic compounds having metal polyanions which have no exopolyhedral ligands at the respective vertices. As a result, it is relatively easy for such polyanions to form metal to metal bonds with atoms of the main metal group and transition metal group. U.S. Pat. No. 5,368,701 entitled Process for Forming Zintl Phases And The Products Thereof provides additional information concerning such materials and their electrical characteristics.
Alternatively, the terms xe2x80x9cZintl phasexe2x80x9d and xe2x80x9cZintl compoundxe2x80x9d may be used to describe a binary compound formed between the alkali or alkaline-earth elements and the main-group elements from group 14 to the right of the xe2x80x9cZintl boundary.xe2x80x9d F. Laves, Naturwissenschaften 29 (1941), p. 244. Some of the features that typify Zintl phases began to be introduced in E. Zintl, W. Z. Dullenkopf, Z. Phys. Chem., Abt. B 16 (1932), p. 183. The definition and both references are taken from John D. Corbett, Chem. Rev. 85 (1985), p. 383-397.
K2SnTe5 compounds are described in Eisenmann et al., Materials Research Bulletin, vol. 18 (1983), pp. 383-387. Tl2GeTe5 compounds are described in Abba Toure et al., Journal of Solid State Chemistry, vol. 84 (1990), pp. 245-252; Marsh, Journal of Solid State Chemistry, vol. 87 (1990), pp. 467-468. Tl2SnTe5 compounds are described in Agafonov et al., Acta Crystallographica C, vol. 47 (1991), pp. 850-852. Zintl phases have been proposed as a place to look for advanced thermoelectric materials. See Sharp, Materials Research Society Symposium Proceedings, vol. 478, pp. 15-24.
Two other researchers have suggested possibly using Zintl phase compounds as thermoelectric materials.
SrSi2xe2x80x94Bruce Cookxe2x80x94Ames National Laboratory
BaSbTe3 and CsSbxTe4xe2x80x94Mercouri Kanatzidisxe2x80x94Michigan State
Some Zintl compounds may be described as clathrate compounds and some clathrate compounds may be described as Zintl compounds. However, many clathrate compounds are not Zintl compounds and many Zintl compounds are not clathrate compounds.
In accordance with teachings of the present invention, the design and preparation of semiconductor materials for fabrication of thermoelectric devices has been substantially improved to provide enhanced operating efficiencies. Examples of such semiconductor materials include, but are not limited to, Tl2SnTe5, Tl2GeTe5, K2SnTe5, Rb2SnTe5 and alloys or mixtures of these compounds.
The present invention provides the ability to obtain increased efficiency from a thermoelectric device having one or more thermoelectric elements fabricated from semiconductor materials having the general formula of X2YZ5 where X represents atoms selected from a group which includes Tl, Cs, K, Na and Rb. Y represents atoms selected from a group which includes Si, Ge and Sn. Z represents atoms selected from a group which includes Te and Se. For some applications X may represent indium (In), copper (Cu) or silver (Ag).
One aspect of the present invention includes fabricating thermoelectric elements from semiconductor materials having a crystal lattice structure composed of a chain of YZ4 tetrahedra linked by a Z atom in a square planar environment. For some applications, Y represents atoms selected from a group which includes silicon (Si), germanium (Ge) and tin (Sn) and, Z represents atoms selected from a group which includes tellurium (Te) and selenium (Se). For still other applications a thermoelectric element may be fabricated in accordance of teachings of the present invention from intermetallic ternary telluride compounds and intermetallic ternary selenide compounds.
Semiconductor materials having crystal lattice structures formed in accordance with the teachings of the present invention optimize selected thermoelectric characteristics of a resulting thermoelectric device. A significant reduction in thermal conductivity (xcexa) may be achieved by establishing relatively long metallic bonds between Te atoms and Tl atoms in the resulting crystal lattice structure. By selecting atoms to form a Zintl compound in accordance with teachings of the present invention, thermal conductivity (xcexa) through the resulting crystal lattice structure may be significantly reduced while at the same time minimizing any reduction in electrical properties (S2"sgr") of the associated semiconductor materials.
Another aspect of the present invention includes forming a crystal lattice structure having two substructures which cooperate with each other to optimize the reduction in thermal conductivity (xcexa) of the associated crystal lattice structure while at the same time minimizing any reduction in electrical properties which results in maximizing the thermoelectric figure of merit for the resulting thermoelectric device. Semiconductor materials having a crystal lattice structure typically associated with Zintl compounds or Zintl phases formed from atoms selected in accordance with teachings of the present invention may demonstrate an order of magnitude decrease in the lattice component of thermal conductivity (xcexag) in comparison with materials previously used to fabricate thermoelectric elements. Thermoelectric devices with thermoelectric elements fabricated from semiconductor materials with such crystal lattice structures may have substantially enhanced thermoelectric operating characteristics and improved efficiencies as compared to previous thermoelectric devices.
In accordance with another aspect of the present invention, both N-type and P-type semiconductor materials with crystal lattice structures similar to those typically associated with Zintl compounds or Zintl phases may be used to fabricate thermoelectric elements for a thermoelectric device. Fabricating a thermoelectric device with such semiconductor materials may substantially enhance the associated operating efficiency as compared to previous thermoelectric devices. Preliminary testing of transport properties indicates that a thermoelectric figure of merit (ZT) of 1.5 or greater is a reasonable expectation for semiconductor materials designed and prepared in accordance with teachings of the present invention. Thermoelectric devices fabricated from such semiconductor materials may be used for cooling, heating, electrical power generation and temperature sensing. Some estimates indicate that providing both N-type and P-type semiconductor materials with a ZT equal to or greater than 1.5 may double the value of the world market for thermoelectric devices.
A ZT of 1.25 for both N-type and P-type materials would make portable freezers based on thermoelectric technology an economically feasible product. Current inexpensive thermoelectric cooling systems are generally limited to operating in refrigeration temperature ranges. Thermoelectric cooling of microprocessors will benefit from enhanced ZT, which is potentially a very large market for thermoelectric devices. Larger increases in ZT could result in widespread use of thermoelectric refrigeration and all-solid state cryocooling. Various ternary intermetallic compounds and ternary semiconductor compounds may be formed in accordance with teachings of the present invention for use in fabricating N-type and P-type thermoelectric elements.